Timing drives generally comprise either an oil-lubricated chain mounted in a compartment connected to the engine oil circuit or a dry-running toothed belt. A timing drive has recently been manufactured wherein either the belt is at least partially dipped in oil at rest, or the oil is conveyed on the belt, e.g. by spraying by means of a specific nozzle or by whipping due to the action of the belt and of the pulleys.
An oil wet belt drive is lighter and causes less vibrations than a chain drive. Furthermore, the belt of an oil wet belt drive requires a lower operating tension than a chain and experimental tests have demonstrated that such drive allows to considerably reduce fuel consumptions and carbon dioxide emissions with respect to a traditional chain drive.
Such advantages may now be obtained also by upgrading or retrofitting engines originally designed for a chain drive. However, a chain drive presents different dimensions than a belt drive. Therefore, in order to upgrade a chain drive to a belt drive, all the components of the latter must be fitted within compartments originally designed for the chain drive. Possibly, some non-substantial parts of the engine may be modified in a marginal manner.
Specifically, an oil wet belt chain may be tensioned by means of an automatic pulley tensioner and this allows to further reduce the loss by friction with respect to chain drives, in which a sliding shoe is used.
However, a compartment designed for accommodating a chain drive presents an internal space which imposes major restrictions to the dimensions of an automatic pulley tensioner both along the axial direction and along the radial direction.
International patent application WO-A1-2006111988 filed by the same applicant describes a tensioner for an oil wet drive comprising a fixed pivot, a disc mounted eccentrically on the pivot and actuated by a tensioning spring and a pulley mounted concentrically on the disc by means of a ball bearing. Furthermore, the tensioner comprises a friction damping device to allow to appropriately dampen the oscillations of the belt also in the presence of oil.
Such a tensioner is compact in an axial direction and is particularly suitable for oil wet belt drives mounted onboard engines designed for operating with such a drive.
However, in the case of upgrade of a chain drive, it is also important to reduce the radial dimension as much as possible.
Furthermore, the belt presents a further reduced width comparable with that of the chain to be mounted within the previously designed compartment. A further reduced width of the belt implies an increase in the axial compacting of the tensioner and a reduction of the extension of the support surfaces, such as for example the cylindrical surface of the pivot.
The loads, on the other hand, are unchanged or tend to increase and require the tensioner to exert a corresponding torque in a reduced space. Furthermore, the specific pressure which loads the components of the tensioners used in an oil wet belt drive requires a particular resistance of the components themselves to avoid excessive wear.